For many years, ink jet technologies which have been developed to produce printheads for ink jet printers and the like have included sub-categories or sub-technologies directed specifically to forming the output ink ejection orifice plate or nozzle plate for controlling the ink drop patterns and ink trajectories onto an adjacent print medium. As is well known to those skilled in the art, these orifice plate technologies include those for making silicon orifice plates, glass orifice plates, plastic orifice plates, and metal orifice plates of many different kinds of materials in each of the latter four types of orifice plate categories. In addition, these metal (e.g. nickel) orifice plate technologies include electroforming and electroplating processes including the fabrication of mandrels for making small geometry precision architecture orifice plates for attachment to thin film printhead substrates.
An example of one highly successful type of thermal ink jet printhead which uses a nickel orifice plate is the St. Helens printhead which has been an integral part of the ThinkJet.TM. disposable thermal ink jet pen sold in large quantities since 1985 by the Hewlett Packard Company of Palo Alto, California. This printhead uses a well known materials set which includes a silicon or quartz substrate upon which thin film layers of silicon dioxide, tantalum aluminum, and aluminum trace material are formed and used to define a pattern of tantalum aluminum heater resistors and electrical interconnects thereto. This thin film printhead structure further includes surface protective layers of silicon nitride and silicon carbide for covering the exposed surfaces of the tantalum aluminum heater resistors and the aluminum conductive trace patterns connected thereto. A polymer barrier is then photolithographically processed on the surface of the silicon nitride/silicon carbide composite layer to define the ink feed channels and ink firing chambers for the heater resistors, and then the nickel orifice plate is precisely aligned and attached to the polymer barrier layer. Orifice openings or "orifii" in the nickel orifice plate are thus carefully aligned with respect to the firing chambers and heater resistors within the thin film printhead structure. For a more detailed description of the above thin film printhead fabrication processes, reference may be made to the Hewlett Packard Journal, Volume 38, No. 5, May 1985. For a more detailed discussion of nickel orifice plate manufacturing per se, reference may be made to U.S. Pat. No. 4,773,971 issued to Si Ty Lam et al, assigned to the present assignee and incorporated herein by reference.
As an alternative to the above printhead fabrication techniques which require not only refined processes for forming the orifice plates, but sophisticated and expensive alignment techniques for attaching the orifice plates to the thin film printhead structures, "orificeless" printheads have been proposed to replace the above described thin film ink jet printheads which have traditionally used orifice plates of various sizes, shapes, and materials. An example of such an orificeless printhead is disclosed in U.S. Pat. No. 4,580,148 assigned to Xerox, and hereinafter referred to as the Xerox patent. The devices disclosed in the Xerox patent operate to develop a thin film of ink on a predefined surface area of the printhead which includes patterns of heater resistors thereon. The thin film of ink is held in place by surface tension, and the heater resistors are fired to eject droplets of ink from this thin film of ink onto an adjacent print media.
However, the disclosed Xerox printhead is operative with several disadvantages, among which include the creation of a vapor bubble in the thin aqueous-based ink film which is characterized by non-uniform nucleation that produces a rough surface area on the bubble and thus poor directionality of the ejected droplets. Thus, the printhead described in the Xerox patent also requires an additional level of design complexity to correct this problem, such as the inclusion of an electrostatic force to provide guidance and directional stability to the ejected droplets. In addition, the printhead device disclosed in the Xerox patent has been designed to use the conservation of momentum of collapsing vapor bubbles exclusively. Therefore, it is not obvious from the teachings of this Xerox patent how to design an ink jet printer that does not include a vapor bubble or that is not thermally driven.